The present invention relates to an alarm system wherein an alarm cord is attached to items to be guarded, such as goods displayed at a store front, or doors or windows, so that an alarm sound can be produced by a disconnection detection signal when the alarm cord is disconnected.
The present applicant has previously proposed another similar alarm system (refer to Utility Model Publication No. 63-47988 in Japan). The alarm system has a composition as is shown in FIG. 2 and the following functions.
The alarm process will be reviewed first. Alarm cord 6 is passed through, for example, the grips 4a, 4b and 4c sequentially with both ends of the cord connected to power source terminal 11 to apply DC voltage Vcc to closed circuit 11 consisting of the alarm cord 6, high resistor 8, protective resistor 9 and LED 10, whereby the alarm system is set to the alarm mode. In this case, the value of the DC voltage Vcc is small, but the value of the high resistor 8 is as large as several M.OMEGA., so that the alarm current is as low as 2-6 .mu.A, and the LED 10 remains unlit. Also, in this alarm mode, both the connection terminals 3 and 7 are at H-level, and thus the output side of inverter circuit 47 is a L-level, and thyristor 48 is left turned off. Inverter circuit 47 is included in a disconnect signal detector 12.
If anyone attempting to steal some goods disconnects the alarm cord 6 or connector 5, this causes the alarm current to be interrupted to cause the level of connection terminal 7 to be changed to L-level whereas the level of the output side of inverter circuit 47 is caused to be changed to H-level to turn on thyristor 48. This causes the output side of the inverter circuit 53 to be set to H-level, whereby the H-level signal is inputted to the alarm generator 21 through diode 54 to drive the first and second oscillation circuits 27 and 33 and transistor 56. This then causes the vibrator (buzzer) 34 of piezoelectric element to be actuated to give an alarm to the store clerks, etc. In this case, even if the disconnected alarm cord 6 or the connecter 5 is reconnected in haste to set the gate of the thyristor to the L-level, the thyristor continues to remain turned on to keep the H-level signal being transmitted to the alarm generator 21, and so the vibrator 34 is kept operating to continue alarming. The current to the thyristor 48 is discontinued only when a movable member 51 of momentary non-locking switch 52 is depressed to cause the movable member to come into contact with constantly opened contact 55, since this causes the thyristor 48 to be turned off to stop the alarm sound.
Secondly, the process of the alarm cancellation necessary when disconnecting the connector 5 to deliver the goods to the customer will be reviewed. First, the movable member of the momentary non-locking switch 52 has to be depressed lightly and released quickly. This causes the movable member 51 of the momentary non-locking switch 52 to contact the constantly opened contact 55, whereby DC voltage Vcc is applied to a charging-discharging circuit 39 of an alarm cancellation signal generator 35 to cause capacitor 38 to charge instantaneously and the movable member 51 to return automatically to the constantly closed contact 49. When the charge of the capacitor 38 is discharged, the partial voltage at both ends of resistor 37, one of discharge resistors 36 and 37 connected in series, actuates the alarm generator 21 through diode 46 in order to indicate the start of cancellation for a short period of time t' (ex. t'=2-3 seconds) only while the partial voltage is at H-level.
Also, at the time of the discharging of the capacitor 38, the output side of Schmitt trigger circuit 45 is set to H-level during a period t (ex. t=4-10 seconds&gt;t') through which the voltage across the discharge capacitors 36 and 37 is at H-level, whereby LED 10 is lit to indicate that the alarm is cancelled, and the output side of the inverter circuit 47 in disconnect signal detector 12 is kept at L-level. Thus, during the period t' regardless of whether the alarm cord 6 is disconnected or not, the thyristor 48 is kept turned off, and so the alarm generator 21 will not be actuated.
Thus, the connector 5 should be disconnected to remove the article 4a, and the connector 5 should be reconnected. After this step, any special resetting operation is not required, since with the expiration of the time period t' the level of the output side of the Schmitt trigger circuit changes to L-level from H-level to restore the alarm state automatically. As explained previously, the vibrator 34 will not be actuated so long as the article 4a is removed and the alarm cord is reconnected during the time period t set by the capacitor 38 and the resistor 36 and 37 of the charging-discharging circuit 39. However, if the alarm cord 6 is not reconnected during the time period t' the vibrator 34 will be actuated. If the vibrator 34 is actuated, the movable member 51 of the momentary non-locking switch 52 should be caused to come into contact with the constantly opened contact 55 by being depressed so that the thyristor is turned off to stop the alarm sound.
As explained in the above, even if disconnecting the article 4a from the alarm cord 6 is delayed, and the alarm sound is produced as a result, the alarm sound can be stopped by operating the momentary non-locking switch, and the cancellation time t can be reset. Thus, even when the alarm sound is started because of delay in disconnecting the article 4a or by error, the alarm sound can be stopped immediately by operating the momentary non-locking switch 52.
When the cancellation time t has elapsed, the level of the cancellation signal outputted from the alarm cancellation signal generator 35 changes to L-level, and the signal is inputted to the gate of the thyristor 48 through the inverter circuit 47 to turn on the thyristor 48. As a result, the level of the signal on the anode side of the thyristor 48 becomes L-level, and the level of the signal becomes H-level in the inverter circuit 53. Then, the signal is inputted to the alarm generator 21 through switching diode 54 to restart the alarm sound. In this case, once the alarm sound is started, it continues, even if the alarm cord 6 and the connector 5 are reconnected, unless the thyristor 48 is turned off.
The alarm system described in the foregoing is capable enough to accomplish its object, but it has been learned that the alarm system still has the following points to be improved for its better performance.
Using the momentary non-locking switch directly for charging the charging-discharging circuit causes a large current to flow through the momentary non-locking switch, thereby requiring the use of large and more durable switches.
Using only one charging-discharging circuit for setting the time t' whereby the start of the alarm cancellation to be indicated and also for setting the time t (&gt;t') whereby the state of the alarm cancellation is to be indicated causes the instabilities of the times for setting t and t'.